This invention relates to the design and manufacture of metallic parts and, more specifically, to a method and a system for improving a part's resistance to stress induced failure.
The combined stress state of a part is defined as the sum total of the applied stresses acting on the part during operation as well as any existing residual stresses. Existing residual stresses are those stresses imparted to the part during manufacturing operations such as casting, forming, welding, heat treatment, machining, etc. The residual stresses in the part may be of such magnitude that, when added to the applied stresses, the part may fail prematurely. A variety of treatments are known to reduce susceptibility to stress induced failure mechanisms. Most of these treatments are employed to mitigate the impact of existing tensile residual stresses in the part by either reducing existing residual tension, such as by heating, or inducing compression in the part. Such compressive stresses are known to influence fatigue and stress corrosion cracking failure by acting in concert with, and in addition to, any applied stress encountered while the part is in service. Common methods of inducing compression in a part include various shot peening techniques, laser shocking or laser shock peening, as well as deep rolling and burnishing. Such methods are typically performed either during or after production of the part.
It is well known that inducing compressive residual stresses in the surface of a part can improve the performance and service life of the part. However, traditional design techniques do not take into account the residual stresses developed in a part during manufacturing. Instead, existing design techniques generally assume that a part will be stress free upon completion. Accordingly, part designs rarely include any consideration of the effect residual stresses will have on the susceptibility of the finished part with regard to stress induced failure. If it is subsequently determined that tensile residual stresses exist in the part and that these residual stresses will have a significant detrimental impact on fatigue life or stress corrosion cracking, one of the aforementioned methods of inducing compression in the part may be performed as a remedial measure. In this way, the effects of tensile residual stresses within the part are reduced or eliminated and the resistance of the part to stress induced failure is improved. Unfortunately, the implementation of such remedial treatments may result in other difficulties.
When compression is induced in a part to alleviate tensile residual stresses it is necessarily accompanied by an equilibrating amount of tension such that the net forces within the volume of the part sum to zero. This equilibrating tension is sometimes problematic. Parts that are designed with extremely close tolerances may not have enough material to accommodate the magnitude of the equilibrating tension associated with the introduction of compression. This can result in failure in the region where the equilibrating tension develops. Further, the compressive residual stresses introduced in the part may buckle or distort the part's dimensions beyond the desired tolerances thereby destroying the viability of the part. In order to counteract this effect, traditional design techniques often call for an excess of material to be added to the part. This addition of extra material operates to reduce the magnitude of equilibrating tension and the distortion that may occur by the introduction of compressive stresses. However, the inclusion of extra part material may add significant weight as well as cost to the finished part. Furthermore, a part containing excess material will typically not be optimized for the given application if component weight or size is a critical concern.
In its conventional form, Haigh Diagrams have been used to demonstrate the interaction between applied mean stress and applied alternating stress, the combination of which effects the fatigue life of a part. Such diagrams have also been used as a means to predict the service life of a part subject to such stresses. Haigh Diagrams are most often used in conjunction with fatigue life prediction functions such as those developed by Soderberg, Goodman, Gerber and Morrow. Such functions describe the fatigue life that can be achieved for a given combination of alternating and mean stresses.
A typical Haigh Diagram, also referred to as a “Goodman” diagram, is commonly used to evaluate the effect of combined stresses in the region where the mean stress is tensile (Smean>0). In limited circumstances, a Haigh Diagram will be developed that will incorporate both tensile and compressive residual stresses in an effort to demonstrate their effect in the combined stress state. For example one such circumstance is disclosed on pages 150 to 154 of “Metal Fatigue in Engineering” by H. O. Fuchs and R. I. Stephens (Copyright 1980 John Wiley & Sons). In this example the compressive quadrant of the Haigh Diagram is shown in conjunction with the Smith-Watson-Topper fatigue life function. However, a method of using the diagram to optimize the fatigue performance of a particular component under given operational conditions is not disclosed. Despite the sporadic inclusion of compressive residual stresses, the exact effect of compressive stresses has not, until now, been shown or taught with respect to the Haigh Diagram.
Consequently, a need exists for a relatively inexpensive method and system for analyzing the impact of tensile residual stresses developed during the manufacturing process of a part and for determining the desired compressive residual stress distributions to be induced in the part to improve the resistance of the part to stress induced failure mechanisms, improve part performance and to effect a significant weight savings.